1. Field of the Invention
The present invention relates to an improvement in a magnetic recording medium.
The present invention specially concerns improvement in the magnetic recording medium specially suitable for magnetic recording tape, magnetic disc, especially for magnetic recording tape for video tape recorder of rotary heads type.
2. Description of the Prior Art
Ferromagnetic thin film type magnetic recording medium, manufactured by forming such a ferromagnetic thin film as of Fe, Co, Ni or their alloys or their oxides on a polyester film, polyimide film or the like high polymer plastic film or non-magnetic metal film or other substrate by means of vacuum deposition, ion plating, spattering, or the like method, is known. These recording media is advantageous in that its recording density can be drastically increased in comparison with the conventional coated-magnetic-film type recording media. For such high density recording, pole gap of the magnetic recording head must be made very small and furthermore surface of the magnetic recording medium must be made very smooth to decrease spacing loss to the utmost extent. However, when the tape surface is too much smoothed, it has been found that squealing of the tape is induced when running around a recording head drum, and therefore the surface state must be finished with a good controlling. Since the thickness of the ferromagnetic thin film is only 100 .ANG.-5000 .ANG., the surface condition of the magnetic recording medium is greatly influenced by the surface configuration of the substrate plastic film. Accordingly there have been many proposals to control the surface conditions of the substrate film surfaces. One of the proposals is disclosed in the Japanese Patent unexamined publication No. Sho 53-107314 disclosing that a magnetic recording medium made by forming a ferromagnetic thin film on a substrate polyester film having thermo-plastic minute granular protrusions has satisfactory tape running characteristic and also a good reproduced picture quality. But this conventional magnetic recording medium has a problem that, when surface roughness of the ferromagnetic recording medium is made to be 300 .ANG. in order to improve the surface smoothness of the magnetic recording medium or attaining a high recording density, stick slips are produced thereby inducing the squealings.
In order to eliminate the squealing of the tape running around the head drum, many proposals have been made, for example, in the Japanese Patent unexamined publications Nos. Sho 53-116115, 53-128685, 54-94574, 56-10455, 56-16937, 57-162126, 57-195321. In these proposed arts, it is proposed that characteristic of touching the head and running is intended to be improved by evenly forming relatively finely roughned surface, for instance, wrinkle shape protrusions, minute earth worm shape protrusions or minute granular shape protrusions, to improve running and head cylinder touching characteristic at the same time. The above-mentioned improvements are very effective in improving the characteristics of head touching, squealing and running in normal temperature and normal moisture condition.
The surface of the recording head especially of a high speed rotating VTR recording heads are likely to be clogged by minute particles of adhesive substance or pasty substance such as fat or pitch which are allergen to the recording head or the magnetic recording medium. For such dirtying phenomenon, the above-mentioned prior disposals are found not effective. For coating type magnetic recording medium, by including minute granules of abrasive to produce minute protrusions on surface of the recording medium, a head-cleaning effect is intended to prevent the dirtying phenomenon. But surface of the aforementioned vacuum deposited, ion plated, spattered or the like manufactured recording medium has relatively moderate radiations of the protrusions, and therefore substantially no polishing effect on the dirtied surface of the recording head. The inventors did many experimental studies and found that magnetic recording medium having a surface with steep mountain-shape protrusions at a rate of large number per area has an effective cleaning function, and made the present invention.